IUPAC Nomenclature of organic compounds [11th class]
IUPAC Nomenclature of organic compounds
The IUPAC system is the most rational and widely used system of nomenclature in organic chemistry. The most important feature of this system is that any given molecular structure has only one IUPAC name and any given name denotes only one molecular structure.
The IUPAC name of any organic compound essentially consists of three parts,i.e.,
1. Word root 2. Suffix and 3. Prefix.
1. Word root. It denotes the number of carbon atoms present in the parent chain. For chains from one to four carbon atoms, special word roots are mused but for chains of five or more carbon atoms Greek number roots are given below :
2. Suffix. There are two types of suffixes :
(i) Primary suffix. A primary suffix is always added to the word root to indicate whether the carbon chain is unsaturated. The three basic primary suffixes are given below :
If the parent chain contains two, three, four or more double or triple bonds, numerically prefixes such as di (for two), trie (for three), tetra (for four),penta (for five) etc. are added to the primary suffix. For example,
1. CH3—CH=CH=CH2
IUPAC Name – But 1,2 di ene
2. CH2=C=C=CH2
IUPAC name – But 1,2,3 tri ene
(ii) Secondary Suffix. A secondary suffix is then added to primary suffix to indicate nature of the functional group present in the organic compound. Secondary suffixes of some important functional groups are given below :
Note – it may be noted that while adding secondary suffix to the primary suffix, the terminal ‘e’ of the primary suffix (I. e.ane, ene, yne) is removed if the secondary suffix begins with vowel but is it retained if the secondary suffix begins with a consonant.
For example :
1. CH3CH2OH
Name – eth an (e) ol — ethanol
(e) is removed because secondary suffix begins with a vowel.
2.CH3CH2CN
Name – prop ane nitril — propanenitrile
(e) is not removed.
3. Prefix.There are two types of prefixes :
(i) Primary prefix. A primary prefix is used simply to distinguish cyclic from acyclic compounds. In case of carbocyclic compounds, cyclo is used immediately before word root. For example :
CH2
/ \
CH2 CH2 Cyclo + pent + ane =cyclobutane
\ / p. Pref. w.Root. p.suff. IUPAC nam.
CH2
(ii) Secondary prefix. In IUPAC system of nomenclature, certain groups are not considered as functional groups but instead are treated as substituents. These are called secondary prefixes and are added immediately before the word root in alphabetical order to denote the side chain or substituent groups.
The secondary prefixes for some groups are given below:
Thus, the complete IUPAC name of a organic compound consists of the following parts :
Secondary prefix + Primary prefix + Word root + Primary suffix + Secondary suffix
*Numbering of carbon atom
Numbering of carbon atoms is the first step for writing the IUPAC name of an organic compound.
Some rules of Numbering are given below :
1. the numbering of carbon chain is always done from that end which is nearer to the bonds and Substituents groups.
2. When both double or triple bonds are present in a molecule, the numbering of carbon chain is always done from that end which is nearer to the double bonds.
3. While the numbering of carbon atoms of an organic compound the lowest set of locants for Substituents is always preferred.
Ex – (i) the set of locants (1,4,5) is preferred over the set (2,3,5).
Some examples :
1 2 3 4 5
(a) CH3—CH—CH2—CH2—CH3
/
CH3
Step 1. Numbering – Numbering is always done from that end which is nearer to Substituent.
Step 2. Secondary prefix – Methyl.
Step 3. Primary prefix – No primary prefix.
Step 4. Word root – pent.
Step 5. Primary suffix – ane (because no double or triple bond).
Step 6. Secondary suffix – no secondary suffix.
Finally the IUPAC name – 2-methyl pentane
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